guest ::
| Home > Publications database > Elementkonzentrationen und Bindungsstruktur von zerstäubungsdeponierten a-Si$_{1-x}$C$_{x}$-Schichten |
| Home > Publications database > Elementkonzentrationen und Bindungsstruktur von zerstäubungsdeponierten a-Si$_{1-x}$C$_{x}$-Schichten |
| Book/Report | FZJ-2019-01419 |
1996
Forschungszentrum Jülich GmbH Zentralbibliothek, Verlag
Jülich
Please use a persistent id in citations: http://hdl.handle.net/2128/21620
Report No.: Juel-3211
Abstract: Sputter-deposited amorphous silicon-carbon films (a-Si$_{1-x}$C$_{x}$, x=0 ... 1) have an important potential as semiconductor material or as first-wall material in fusion reactors. A particular feature of sputter-deposition is that the mean energy ofthe deposited atoms is in the range of a few electronvolts. In this thesis some basic properties of such a-Si$_{1-x}$C$_{x}$ films are explored, e.g. the atomic density, the atomic concentration, and the behaviour of the Si- and C-bonds. In the second part the influence of oxygen exposure during growth is investigated. X-ray Photoelectron Spectroscopy (XPS) is mainly used for analysis. Additional information wasobtained from Rutherford Backscattering Spectroscopy (RBS), Nuclear Reaction Analysis (NRA), Auger Electron Spectroscopy (AES), X-ray induced AES (XAES), Energy Dispersive X-ray spectroscopy (EDX), and InfraRed transmission spectroscopy (IR). To understand the properties of the a-Si$_{1-x}$C$_{x}$ material it is important to study the structure of the prevailing bonds. It is known that silicon and carbon in sp$^{3}$-hybridization have four covalent bonds in a tetragonal co ordination. SiC behaves in a similar manner, whereby atoms of the one species are surrounded by atoms of the other species in a tetragonal configuration. It has to be kept in mind, that carbon can exist in a diamond (sp$^{3}$) as well as in a graphitic (sp$^{2}$ modification. The crystalline forms of C, Si and SiC are quite well known, but the amorphous forms of these elements and their mixture are less understood and more complex. Their characterisation is the main topic of this thesis. Sputter-deposited a-Si$_{1-x}$C$_{x}$ films were produced by ion bombardment of abipartite target, consisting of a Si-wafer and a piece of graphite. The film composition was varied bytranslating the target so that the ion beam was moved across the Si/C-boundary. In order to determine the atomic concentration of these films by XPS it is necessary to know the atomic density N. To determine N, a particular electron energy loss (to plasmons) was measured. This energy loss shifts continuously from $\Delta$E=17.8 eV for pure silicon to $\Delta$E=29.0 eV for pure carbon. Because of the connection between the plasmon energy and the density of valence-electrons (n$_{e} \sim \Delta$E$^{2}$) and under the assumption that silicon and carbon have four valence-electrons, the atomic density can be calculated. It was found, that for puresputter-deposited silicon N=5.2 · 10$^{22}$ cm$^{-3}$, which is almost the same value as for ordinary crystalline or amorphous silicon. For sputter-deposited carbon a value of N=1.6 · 10$^{23}$ cm$^{-3}$ was found, which is nearly the value for diamond. This means, that there is a preference for a sp$^{3}$-hybridization rather than for the graphitic structure (sp$^{2}$). Considering the preparation-geometry the concentration and the density were calculated. These theoretical values agree with the XPS-results if the relative sputtering-yield carbon to silicon is set to Y$_{c}$/Y$_{si} \approx$ 0.4. To check the accuracy ofthe atomic concentration some further experiments were performed. By means of RBS and NRA (the results of which are independent of the chemical bonding) the same values ofthe density were found. Additionally, the density of some samples was determined by the direct measurement of their volume and mass, again with similar results. The chemical order of the a-Si$_{1-x}$C$_{x}$ films was investigated. Three models were developed to explain the energy-shifts and -widths of the C1s- and Si2p-peaks in the XP-Spectra, whereby it was assumed that the atomic coordination number is four. It was shown, that there is a preference for the bonding between Si- and C-atoms rather than between Si and Si or C and C, so that the system tries to make as many Si-C-bonds as possible. It was found, that every minority atom is surrounded by four majority atoms. This result is in accordance with IR measurements, which additionally show that the films are amorphous (peakwidth and -position). Additional oxygen-exposure during filrn-growth was investigated in the last part of this thesis. It was noticed, that there is a tendency for oxygen to react preferentially with silicon in the case of high carbon contents. Bonding between carbon and oxygen is hardly found. Bonds between silicon and oxygen are seen as suboxides (SiO$_{z}$ z<2). It is expected that the growth-rate of the a-Si$_{1-x}$C$_{x}O_{y}$ films plays an important role for this behaviour. Because the sputtering-yield-ratio of C to Si is smaller than one, the growth-rate decreases with increasing carbon content. This on the other hand increases the reaction-time of oxygen with the surface. One might also speculate, that carbon could work as a catalyst for the Si-O reaction.
|
The record appears in these collections: |
PDF
PDF (PDFA)